The widely used tetrazolium dye, MTT, has several advantages as a vital stain in the identification of viral plaques. First, since the yellow colored dye MTT stains live cells dark blue, viral plaques can be counted without removal of the phenol red containing agar overlay. Secondly, the high contrast between live and dead cells afforded by MTT permits one to readily detect small plaques at an early time after infection. Thirdly, since cells intensely stained with MTT remain viable, MTT vital staining can be used to isolate cells which are either transformed or resistant to viruses. Thus, MTT vital staining should be useful in several types of viral plaque assays.